1. Field of the Invention
The invention generally relates to flow control apparatus for subterranean wells connected by wellbore with the surface and, more particularly, relates to a safety valve which may be set by packer for simultaneously controlling fluid flow through an annulus and tubing string connecting the subterranean well with the surface through the well bore.
2. Description of the Related Art
In a number of applications, flow control apparatus must serve to simultaneously allow or restrict dual flows, for example, simultaneous upward and downward flows, through separate flow paths within a wellbore. One example of such an application is natural gas storage wells. A natural gas storage well is a subterranean cavern formed within a salt dome or other similarly non-permeable cavernous geologic structure. These underground caverns are connected with the surface through a wellbore drilled from the surface to the cavern. The wellbore is typically cased with a casing string from the surface to the cavern, and one or more tubing strings may be run within the cased wellbore for flow of fluids from and to the cavern to the surface.
In operation of a gas storage well, previously produced gas is pumped under pressure down into the subterranean cavernous storage structure. Because the gas pumped into the storage well is pressurized, the well must include some means for containing the pressurized gas within the well and some means for allowing select flow of the gas from the underground cavern, through the wellbore, to the surface, when retrieval is desired.
In certain gas storage wells, it is desired that two different fluids, for example, gas and water, be simultaneously pumped into and out of, respectively, the storage well. These wells will typically have what is called a syphon tubing string extending from the surface through the wellbore to near the bottom of the storage well within the underground cavernous structure. Gas is generally pumped into the storage well through the annulus space in the wellbore formed between the syphon tubing string and the well casing. Water is generally retrieved up the syphon string. By selectively controlling flows of gas and water into and out of these gas storage wells, a somewhat constant pressure may be maintained within the storage well by simultaneously varying the gas volume and the water volume of the storage well. For instance, as gas is retrieved through the annulus between the syphon tubing string and the wellbore casing out of the storage well, water is pumped into the storage well through the syphon tubing string in order to maintain pressure by reducing volume available for gas storage within the storage well. The reverse process, gas injection into and water retrieval from the storage well, can also maintain storage well pressure in a similar manner.
In these storage wells and other similar storage arrangements, some means is necessary for simultaneously controlling flow of gas and other fluid into and out of the storage well, as previously mentioned. In the past, safety valves have been placed at the surface in the wellhead (or "Christmas tree") which tops the wellbore. The wellheads in these prior arrangements also have contained at least some, if not all, of the actuating parts of those safety valves. Though these safety valves have proven somewhat effective in controlling flows of gas and other fluids in and out of storage wells, location of these valves and the actuating parts within the wellhead at the surface presents certain problems. In particular, if the wellhead is ever damaged, the safety valves may be damaged or otherwise affected, possibly causing loss of the safety system. If the flow control safety system is lost, gas within the storage wells can escape, often with disastrous effects.
The present invention provides a means for controlling dual flows into and out of pressurized storage chambers, such as underground storage wells. The invention is an improvement over the art because the flow control mechanism is not dependent upon occurrences at the surface, such as at the wellhead. Thus, any problems with damage to the wellhead will not affect the operation of the mechanism as a safety device. Additionally, the flow control mechanism allows for two separate flow paths with simultaneous control of flow within those paths. The mechanism additionally provides for equalization of pressure between the flow paths when the storage chamber is shut in. Even further, the mechanism may be run on coil tubing and may be run, set, put on line, and retrieved under pressure, allowing for practical operation without requiring extreme measures. Finally, the mechanism is contained in a single device and so results in reduced expense and simplification of installation and operation of the device.